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From:   Marco Elver <elver@google.com>
Date:   Thu, 9 Sep 2021 13:43:44 +0200
Message-ID: <CANpmjNPBdx4b7bp=reNJPMzSNetdyrk+503_1LLoxNMYwUhSHg@mail.gmail.com>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Nathan Chancellor <nathan@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev, Nick Desaulniers <ndesaulniers@google.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
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On Thu, 9 Sept 2021 at 13:00, Arnd Bergmann <arnd@kernel.org> wrote:
> On Thu, Sep 9, 2021 at 12:54 PM Marco Elver <elver@google.com> wrote:
> > On Thu, 9 Sept 2021 at 07:59, Christoph Hellwig <hch@infradead.org> wrote:
> > > On Wed, Sep 08, 2021 at 11:58:56PM +0200, Marco Elver wrote:
> > > > It'd be good to avoid. It has helped uncover build issues with KASAN in
> > > > the past. Or at least make it dependent on the problematic architecture.
> > > > For example if arm is a problem, something like this:
> > >
> > > I'm also seeing quite a few stack size warnings with KASAN on x86_64
> > > without COMPILT_TEST using gcc 10.2.1 from Debian.  In fact there are a
> > > few warnings without KASAN, but with KASAN there are a lot more.
> > > I'll try to find some time to dig into them.
> >
> > Right, this reminded me that we actually at least double the real
> > stack size for KASAN builds, because it inherently requires more stack
> > space. I think we need Wframe-larger-than to match that, otherwise
> > we'll just keep having this problem:
> >
> > https://lkml.kernel.org/r/20210909104925.809674-1-elver@google.com
>
> The problem with this is that it completely defeats the point of the
> stack size warnings in allmodconfig kernels when they have KASAN
> enabled and end up missing obvious code bugs in drivers that put
> large structures on the stack. Let's not go there.

Sure, but the reality is that the real stack size is already doubled
for KASAN. And that should be reflected in Wframe-larger-than.

Either that, or we just have to live with the occasional warning (that
is likely benign). But with WERROR we're now forced to make the
defaults as sane as possible. If the worry is allmodconfig, maybe we
do have to make KASAN dependent on !COMPILE_TEST, even though that's
not great either because it has caught real issues in the past (it'll
also mean doing the same for all other instrumentation-based tools,
like KCSAN, UBSAN, etc.).